1. Field of the Invention
The present invention generally relates to an optical scanner apparatus, and more particularly to an imaging device of an optical scanning module to prevent the light dispersion effect and obtain the better mtf (modulation transfer function).
2. Description of the Prior Art
Scanner devices, such as flatbed scanners, are well known in the art and produce machine-readable image data signals that are representative of scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by a scanner may be used by personal computer to reproduce an image of the scanned object in a suitable display device, such as a CRT (cathode ray tube) or a print.
Flatbed scanners are widely used with computer systems for converting printed data into image signals. An optical scanning module is the most important component of the flatbed scanner. The scanning module commonly comprises a housing with an opening for receiving light transmitted from a scanning object, an optical sensor such as CCD (charge coupled device) installed inside the housing for converting the transmitted light into corresponding image signals, and a plurality of lenses for focusing the transmitted light onto the optical sensor.
A typical flatbed scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (commonly referred to as a “scan region”), whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a “scan line”) onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object, usually by moving the illumination and optical systems with respect to the object.
The illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array if light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to focus the image of the illuminated scan line onto the surface of the detector. Alternative, a “contact image sensor” (CIS) may be used to collect and focus light from the illuminated scan region onto the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprises an array of an individual cells or “pixels”, each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon.
Referring to FIG. 1A, the reference number 100 is scanning an object; reference number 102 is reflected light which is illuminated the document to form a reflected light; reference number 104 is a conventional lens-imaging device of an optical scanning module; and reference number 106 is an optical sensor. In conventional scanning method of an optical scanner apparatus, a reflected light 102 from the scanning object 100 is transmitted to the lens-imaging device 104 to the optical sensor 106, and an optical imaging (not shown in FIG. 1A) is focused on the optical sensor 106. In the conventional optical scanning module, the imaging data is focused on the optical sensor 106 which is according to the theorem of the formation of image, the reflected light 102 is transmitted from the scanning object 100 to the lens-imaging device 104, the imaging data is communicated to the optical sensor 106. Then, the imaging data is treated and output to the output device, such as a print or monitor (not shown in FIG. 1A).
The response distribution curve diagram of the FIG. 1B is formed that according to the FIG. 1A. According to the theorem of the formation of image, the reflected light point 102A, 102B, and 102C of the scanning object 100 and transmitted to the lens-imaging device 104 to the optical sensor 106 to form a response distribution curve diagram 200. Due to the light dispersion effect, the two ends 202A and 202B of the response distribution curve of the response distribution curve diagram 200 are lower than the response distribution curve that is formed from the light 102C (straight light) and other stream of light (not shown in FIG. 1B) between the ends 202A and 202B. Furthermore, due to the light dispersion effect, the imaging data cannot focus completely on a point such that the resolution of the imaging data on the two ends 202A and 202B is bad. Therefore, the resolution of the scanning object will be degraded.
In addition, the optical scanner apparatus with a lens-imaging device as a formation of image is used for scanning the scanning object 100, the optical image of the scanning object 100 is focused on the optical sensor 106 and converted the analogy signal into digital signal by ADC (analogy digital convert system). Therefore, in order to obtain a higher resolution, the number of pixel for CCD 106 must be increase. However, the cost of the CCD 106 is proportional to the square of the number of pixels. When the higher resolution is required that will cause the cost of CCD 106 is a great part of the production of the optical scanner. Furthermore, the lens-imaging device exhibits a plane to scan the scanning object 100, and the MTF (modulation transfer function) is poor when the optical sensor 106 is diverged from the plane of the lens-imaging device 104.